Data Link Issues Relates to Lab 2 This

Data Link Issues Relates to Lab 2. This module covers data link layer issues, such as local area networks (LANs) and point-to-point links, Ethernet, and the Point-to-Point Protocol (PPP). 1

TCP/IP Suite and OSI Reference Model • The TCP/IP protocol stack does not define the lower layers of a complete protocol stack • In this lecture, we will address how the TCP/IP protocol stacks interfaces with the data link layer and the MAC sublayer 2

Data Link Layer • The main tasks of the data link layer are: • Transfer data from the network layer of one machine to the network layer of another machine • Convert the raw bit stream of the physical layer into groups of bits (“frames”) 3

Types of Networks • There are two types of communication networks: – Broadcast Networks: All stations share a single communication channel – Point-to-Point Networks: Pairs of hosts (or routers) are directly connected • Typically, local area networks (LANs) are broadcast and wide area networks (WANs) are point-to-point 4

Local Area Network • Local area networks (LANs) typically connect computers within a building or a campus • Almost all LANs are broadcast networks • Typical topologies of LANs are bus or ring or star • We will work with Ethernet LANs. Ethernet has a bus or star topology. 5

MAC and LLC • In any broadcast network, the stations must ensure that only one station transmits at a time on the shared communication channel • The protocol that determines who can transmit on a broadcast channel are called Medium Access Control (MAC) protocol • The MAC protocol are implemented in the MAC sublayer which is the lower sublayer of the data link layer • The higher portion of the data link layer is often called Logical Link Control (LLC) 6

IEEE 802 Standards • IEEE 802 is a family of standards for LANs, which defines an LLC and several MAC sublayers 7

Ethernet • Speed: • Standard: 10 -1000 Mbps 802. 3, Ethernet II (DIX) • Most popular physical layers for Ethernet: • • • 10 Base-T 10 Base 2 100 Base-TX 100 Base-FX 10 Mbps Twisted Pair Thin Ethernet: 10 Mbps thin coax cable 100 Mbps over Category 5 twisted pair 100 Mbps over Fiber Optics 1 Gbps over Fiber Optics 8

Bus Topology • 10 Base 5 and 10 Base 2 Ethernets have a bus topology 9

Star Topology • With 10 Base-T, stations are connected to a hub in a star configuration 10

Ethernet Hubs vs. Ethernet Switches • An Ethernet switch is a packet switch for Ethernet frames • Buffering of frames prevents collisions. • Each port is isolated and builds its own collision domain • An Ethernet Hub does not perform buffering: • Collisions occur if two frames arrive at the same time. Hub Switch 11

Ethernet and IEEE 802. 3: Any Difference? • On a conceptual level, they are identical. But there are subtle differences that are relevant if we deal with TCP/IP. • “Ethernet” (Ethernet II, DIX) • An industry standards from 1982 that is based on the first implementation of CSMA/CD by Xerox. • Predominant version of CSMA/CD in the US. • 802. 3: • IEEE’s version of CSMA/CD from 1985. • Interoperates with 802. 2 (LLC) as higher layer. • Difference for our purposes: Ethernet and 802. 3 use different methods to encapsulate an IP datagram. 12

Ethernet II, DIX Encapsulation (RFC 894) 13

IEEE 802. 2/802. 3 Encapsulation (RFC 1042) 14

PPP - Point-to-Point Protocol • The PPP protocol is a data link protocol for transmission on a serial link • Use of PPP today: – Dial-in or DSL access to Internet – Routers connected by point-topoint links • Main purpose of PPP is encapsulation of IP datagrams • PPP was proposed in 1992; a predecessor of PPP was the Serial Link IP (SLIP) protocol 15

PPP - IP encapsulation • The frame format of PPP is similar to HDLC and the 802. 2 LLC frame format: 16

PPP Other than a framing method PPP provides: – The link control protocol (LCP) which is responsible for establishing, configuring, and negotiating a data-link connection • LCP is specified in RFC 1331. – For each network layer protocol supported by PPP, there is one network control protocol (NCP) • The NCP for IP is specified in RFC 1332 17